Inverters

Abstract

We describe a photovoltaic power conditioning unit for delivering power from multiple photovoltaic panels to an ac mains power supply output, comprising: a dc input for receiving power from multiple photovoltaic panels; an ac output for delivering ac power to the ac supply; a bank of electrolytic energy storage capacitors for storing energy from the dc source for delivery to the ac supply; a dc-to-ac converter coupled to the ac output and having an input coupled to the bank for converting energy stored in the bank to ac power for the ac supply; and further comprising: a plurality of sense and control circuits, one for each capacitor in the bank, wherein each circuit is coupled in series with a capacitor, and is configured to disconnect the associated capacitor from the bank upon detection of a current flow through the associated capacitor of greater than a threshold current value.

The invention claimed is: 
     
       1. A photovoltaic power conditioning unit for delivering power from one or more photovoltaic panels to an ac mains power supply, the power conditioning unit comprising:
 a dc input for receiving power from the one or more photovoltaic panels; 
 an ac output for delivering ac power to the ac mains power supply; 
 a first bank of energy storage capacitors for storing energy from the dc input for delivery to the ac mains power supply, wherein the first bank of energy storage capacitors comprises a mixture of electrolytic and non-electrolytic energy storage capacitors; 
 a dc-to-ac converter coupled to the ac output and having an input coupled to the first bank of energy storage capacitors, the dc-to-ac converter for converting energy stored in the first bank of energy storage capacitors to ac power, and providing the ac power to the ac mains power supply; 
 a temperature sensor for sensing a temperature of the photovoltaic power conditioning unit; and 
 a sense and control circuit for disconnecting one or more of the electrolytic energy storage capacitors in the first bank of energy storage capacitors. 
 
     
     
       2. The photovoltaic power conditioning unit of  claim 1  wherein the sense and control circuit includes a plurality of sense and control circuits, one for each of the electrolytic energy storage capacitors in the first bank of energy storage capacitors, wherein each sense and control circuit of the plurality of sense and control circuits is coupled in series with a respective one of the electrolytic energy storage capacitors, and wherein each sense and control circuit of the plurality of sense and control circuits is configured to disconnect the associated electrolytic energy storage capacitor from the first bank of energy storage capacitors upon detection of a current flow through the associated electrolytic energy storage capacitor that is greater than a threshold current value. 
     
     
       3. The photovoltaic power conditioning unit of  claim 2 , wherein each of the plurality of the sense and control circuits comprises a low-pass filter in a sense-control loop to attenuate a high frequency component from switching of the dc-to-ac converter when detecting the current flow though the associated electrolytic energy storage capacitor. 
     
     
       4. The photovoltaic power conditioning unit of  claim 3 , wherein the low-pass filter has a corner frequency of at least 1 KHz. 
     
     
       5. The photovoltaic power conditioning unit of  claim 2  further comprising a capacitor technology control circuit coupled to one or more of each of the plurality of sense and control circuits to control selective connection of each associated electrolytic energy storage capacitor to an internal bus of the first bank of energy storage capacitors. 
     
     
       6. The photovoltaic power conditioning unit of  claim 5 , further comprising at least one temperature sensor, wherein the selective connection of each associated electrolytic energy storage capacitor comprises disconnection of each associated electrolytic energy storage capacitor when a temperature sensed by the temperature sensor is below a threshold temperature value. 
     
     
       7. The photovoltaic power conditioning unit of  claim 5 , wherein the selective connection of each associated electrolytic energy storage capacitor comprises disconnection of the each associated electrolytic energy storage capacitor during a start-up period of the power conditioning unit. 
     
     
       8. The photovoltaic power conditioning unit of  claim 1  wherein the sense and control circuit comprises a senseFET. 
     
     
       9. The photovoltaic power condition unit of  claim 1 , wherein the sense and control circuit comprises a fuse, and wherein a disconnection of the fuse is dependent upon a time-averaged square of a current flowing onto or off the first bank of energy storage capacitors being greater than a threshold value. 
     
     
       10. The photovoltaic power conditioning unit of  claim 1 , wherein a single energy storage capacitor of the first bank of energy storage capacitors has a capacitance that is sufficient for an ac ripple voltage component at a frequency of the ac mains power supply to pull an instantaneous voltage on the first bank of energy storage capacitors below zero when the power conditioning unit is operating at a maximum rated ac output power. 
     
     
       11. The photovoltaic power conditioning unit of  claim 1  further comprising a second bank of energy storage capacitors, and a capacitor bank control circuit to selectively connect one of the first and the second banks of energy storage capacitors to the dc-to-ac converter. 
     
     
       12. The photovoltaic power conditioning unit of  claim 11 , wherein the capacitor bank control circuit is configured to time-multiplex a use of the first and the second banks of energy storage capacitors. 
     
     
       13. The photovoltaic power conditioning unit of  claim 12 , wherein the capacitor bank control circuit is further configured to avoid the use of the first bank of energy storage capacitors responsive to a detection that all electrolytic energy storage capacitors in the first bank of energy storage capacitors have failed, and to avoid the use of the second bank of energy storage capacitors responsive to a detection that all electrolytic energy storage capacitors in the second bank of energy storage capacitors have failed. 
     
     
       14. The photovoltaic power conditioning unit of  claim 1  further comprising a dc-to-dc converter coupled between the dc input and the first bank of energy storage capacitors and configured to increase a dc voltage from the one or more photovoltaic panels for the dc-to-ac converter. 
     
     
       15. The photovoltaic power conditioning unit of  claim 1 , wherein the power conditioning unit has a single power conversion stage. 
     
     
       16. The photovoltaic power conditioning unit of  claim 1 , wherein the power conditioning unit has a maximum rated ac output power of less than 600 Watts. 
     
     
       17. The photovoltaic power conditioning unit of  claim 1 , wherein the first bank of energy storage capacitors comprises less than 20 energy storage capacitors. 
     
     
       18. The photovoltaic power conditioning unit of  claim 1 , wherein one or more energy storage capacitors in the first bank of energy storage capacitors are disconnected based on a sensed temperature from the temperature sensor. 
     
     
       19. A method of enhancing the reliability of a photovoltaic power conditioning unit for delivering power from one or more photovoltaic panels to an ac mains power supply, the power conditioning unit comprising:
 a dc input for receiving power from the one or more photovoltaic panels; 
 an ac output for delivering ac power to the ac mains power supply; 
 a bank of energy storage capacitors for storing energy from the dc input for delivery to the ac mains power supply, wherein the bank of energy storage capacitors comprises a mixture of electrolytic and non-electrolytic energy storage capacitors; 
 a dc-to-ac converter coupled to the ac output and having an input coupled to the bank of energy storage capacitors, the dc-to-ac converter for converting energy stored in the bank of energy storage capacitors to ac power and providing the ac power to the ac mains power supply; the method comprising: 
 sensing a temperature of the photovoltaic power conditioning unit; and 
 disconnecting one or more energy storage capacitors from the bank of energy storage capacitors based on the sensed temperature. 
 
     
     
       20. The method of  claim 19 , wherein one or more energy storage capacitors in the bank of energy storage capacitors are disconnected when the sensed temperature is below a threshold temperature.